1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus.
2. Description of the Related Art
Conventionally, there are known to be image forming apparatuses that form electrostatic latent images on a photosensitive member by using a rotating polygonal mirror to deflect a light beam emitted from a light source and scanning the photosensitive member with the deflected light beam. This kind of image forming apparatus includes an optical sensor (beam detection (BD) sensor) for detecting the light beam deflected by the rotating polygonal mirror, and the optical sensor generates a synchronization signal upon detecting the light beam. By causing the light beam to be emitted from the light source at a timing determined using the synchronization signal generated by the optical sensor as a reference, the image forming apparatus aligns the writing start positions for the electrostatic latent image (image) in the direction (main scanning direction) in which the light beam scans the photosensitive member.
Also, there are known to be multi-beam image forming apparatuses that include multiple light emitting elements as a light source for emitting multiple light beams that each scan different lines on the photosensitive member in parallel in order to realize a higher image formation speed and higher resolution images. With this kind of multi-beam image forming apparatus, a higher image formation speed is realized by scanning multiple lines in parallel using multiple light beams, and higher resolution images are realized by adjusting the interval between the lines in the sub-scanning direction.
Japanese Patent Laid-Open No. 2008-89695 discloses an image forming apparatus that includes multiple light emitting elements as a light source and is capable of adjusting the resolution in the sub-scanning direction by performing rotational adjustment of the light source in the plane in which the light emitting elements are arranged. This kind of resolution adjustment is performed in the step of assembling the image forming apparatus. Japanese Patent Laid-Open No. 2008-89695 discloses a technique for suppressing misalignment in the writing start positions in the main scanning direction for the electrostatic latent image that occur due to light source attachment errors in the assembly step. Specifically, the image forming apparatus uses a BD sensor to detect light beams emitted from a first light emitting element and a second light emitting element and generates multiple BD signals. Furthermore, the image forming apparatus sets a light beam emission timing for the second light emitting element relative to the light beam emission timing for the first light emitting element based on the generation timing difference between the generated BD signals. This compensates for light source attachment errors in the assembly step and suppresses misalignment in the writing start positions for the electrostatic latent image between the light emitting elements.
Also, there is known to be a technique of shorting, in an image forming apparatus, the period from when image formation processing is started until when a recording sheet on which an image has been formed is discharged to the greatest extent possible, thereby starting a polygon motor at an earlier time in order to obtain print output somewhat earlier. For example, Japanese Patent Laid-Open No. 2009-297917 discloses an image forming apparatus which, when a document is set, starts a polygon motor without turning on a light emitting element (laser diode) and controls the rotation speed of the polygon motor so as to be constant. Upon receiving input of a job in a state where the polygon motor is rotating at a stable rotation speed, this image forming apparatus turns on the light emitting element in order to cause a BD sensor to output a BD signal. Furthermore, the image forming apparatus starts an image forming operation at a time when the cycle of the BD signals output from the BD sensor reaches a cycle proportional to a target number of rotations of the polygon motor. Thus, the image forming apparatus disclosed in Japanese Patent Laid-Open No. 2009-297917 generates BD signals in non-image-forming periods, in which image formation is not performed.
However, the following problems are present in the method of, in an image forming apparatus including multiple light emitting elements as a light source, measuring the generation timing difference between BD signals generated by a BD sensor as described above.
If it is possible to execute multiple times of measuring the generation timing difference (time interval) between two BD signals corresponding to light beams emitted from first and second light emitting elements in a non-image-forming period, the measurement accuracy can be improved by averaging the obtained measurement values. In general, the length of a non-image-forming period changes depending on the size of the sheet used in image formation, adjustment operations performed in the non-image-forming period, and the like. However, the number of times of measuring the time interval between BD signals performed in a non-image-forming period has conventionally been set according to the shortest non-image-forming period, and therefore there have been cases where a number of measurement values sufficient for achieving the required measurement accuracy cannot be obtained. In particular, as shown in FIG. 9, when a polygon mirror starts to rotate, the temperature in the image forming apparatus (optical scanning apparatus) changes dramatically. In this case, if the time needed to obtain the number of measurement values necessary for averaging increases in length, the average value of the BD interval measurement results will have a greater error. For this reason, in order to improve the measurement accuracy while following this kind of temperature change, it is desirable to execute a greater number of times of measurement in a non-image-forming period.